Differential pressure process flow meters, long known in the art, measure the rate of flow of a liquid or gas by calculating the difference in pressure between a measurement taken at a high pressure tap upstream of a process flow device, and a low pressure tap downstream of the process flow device. When the primary flow element used is an orifice plate within a fluid carrying conduit such as a pipe, commonly used pressure tap types include: flange taps, corner taps, D & D/2 taps, and pipe taps. Although the tap configuration and axial distances from the orifice plate differ, all of these pressure tap types are located at, or affixed to, the pipe wall. While the accuracy of such taps is good for well-developed fluid flow and stable pipe conditions, in certain conditions pipe flow disturbances may occur along the pipe wall, interfering with the proper operation of these common tap types and creating disturbances in the pressure signals they generate.
Pipe flow disturbances within a pipe can have a number of causes including an upstream fitting such as an elbow in the pipe. The commonly used apparatus to correct for these flow disturbances is a flow straightener, of the type disclosed in U.S. Pat. No. 5,596,152 or apparatus similar to the flow conditioner described in U.S. Pat. No. 3,733,898. A more complex apparatus, employing a plurality of elongated openings arranged in a predetermined pattern in a transversely disposed plate within the fluid-conducting pipe, together with a computer to deal with the necessary algorithms, is disclosed in U.S. Pat. No. 5,295,397. Yet another device for reducing the adverse measuring effects of a distorted velocity profile is known as the piezometer ring. This appliance may surround the orifice on both sides of the orifice plate and, by means of a plurality of circumferentially disposed pressure sensing ports, averages pressures around the upstream and downstream sides of the orifice. Although not shown with an orifice plate flow meter, a piezometer type of averaging sensor is described generally in U.S. Pat. No. 5,279,155.
Flow straighteners, conditioners, computers and piezometers are moderately effective to properly condition the velocity profile for introduction to an orifice plate flowmeter, or average the asymmetric velocity of the flow, but have the disadvantage of adding separate and additional components to the process piping with the attendant initial cost, pressure drop in the fluid, and increased maintenance requirements.
Improved means of conditioning fluid flow that do not experience the aforementioned problems have been developed. One such means is the Rosemount Conditioning Orifice Plate technology, set forth in U.S. Patent Appl. No. 20030188586, Averaging Orifice Primary Flow Element. The Averaging Orifice Plate incorporates a plurality of eccentrically disposed apertures in the surface of the plate to achieve an averaging of the differential pressures across the plate despite velocity profile distortion of the fluid presented to the primary flow element. However, higher beta ratio conditioning orifice plates using common pressure sensing ports spaced some distance upstream and downstream from the plate, are still susceptible to performance errors in disturbed flow conditions due to disturbances in the pipe to which pressure sensing ports are attached or the impact of the disturbances on the pressure sensing ports themselves.
Accordingly, the primary object of the present invention is to maintain the benefits of utilizing conditioning orifice plate differential pressure flow meters while minimizing performance errors which may be caused by flow disturbances to pressure ports, especially at higher beta ratios.
Other and further objects, features and advantages of the invention will become apparent from the following description of embodiments of the invention, taken in conjunction with the accompanying drawings.